Add some easy macros

[bennn]

• a print (or assert) that optionally expands into nothing
• define + provide a parameter,
  • maybe also typed
  • maybe also with name conventions (start with current-, start and end with *)
• reprovide-like macro (https://github.com/AlexKnauth/reprovide-lang)
• simple rackunit abbreviations (https://github.com/bennn/rackunit-abbrevs)